This invention relates to metal-matrix composite materials, and, more particularly, to the fabrication of articles from such materials by melting and casting.
In one form of a metal-matrix composite material, a reinforcement phase is embedded in a metal matrix. The reinforcement is typically equiaxed or elongated particles of a ceramic phase such as aluminum oxide or silicon carbide, and the matrix is a pure metal or alloy such as aluminum. The particle phase and the matrix metal phase each retains its separate physical and chemical identity in the composite material, and each phase contributes to the properties of the final composite material.
Several techniques are available to make useful articles of such materials. In one approach, the metallic matrix material is melted and wet to the particles, either by mixing or infiltration. The wetted mixture, in the form of a slurry of the wetted ceramic particles in a molten matrix, is then cast directly into molds in the case of the mixing approach, or diluted and then cast into molds in the case of the infiltration approach.
For some applications, the metal-matrix composite material is cast into foundry ingots at one location and shipped to the facility of a foundry user. The foundry user remelts the matrix portion of the foundry ingots, forming a remelted slurry, by heating the ingots to a temperature above the melting point of the matrix material, and then casts the remelted slurry into molds that define the shape of the final article. During the remelting operation, the remelted composite material sometimes is held at elevated temperature for several hours before casting, due to the logistics of the casting operation.
For some foundry casting operations, the remelted metal-matrix composite material must be reheated in a furnace to temperatures well above the melting point of the metal matrix. If this temperature is sufficiently high that the ceramic reinforcement chemically reacts with the matrix material to a significant degree, the resulting reaction product generally increases the viscosity of the slurry. The slurry of increased viscosity is more difficult to cast than it is prior to the chemical reaction, impairing the ability to cast many articles. Additionally, the reaction product cast into the final product may adversely affect its properties.
Several solutions to this problem are known. In one, the surfaces of the particles are coated or treated in-situ to reduce their reactivity. In another, specific matrix alloys having reduced reactivity are selected. In yet another, the remelt temperature is limited so as to reduce the extent of chemical reaction. These various approaches are workable in some circumstances, but not in others due to technical or cost issues.
There is a need for an improved approach to the remelt processing of castable metal-matrix composite materials. The present invention fulfills this need, and further provides related advantages.